Paper making retention system of bentonite and a cationic galactomannan

ABSTRACT

The invention concerns a novel method for making paper based on sheet cellulose fibre, wherein a novel retention system comprising a suspension of bentonite and a cationic galactomannan is used to improve in particular the retention of the incorporated mineral fillers. The invention also concerns a method for making paper using a retention system which substantially improves draining.

This application is an application under 35 U.S.C. Section 371 ofInternational Application Number PCT/FR99/00969 filed on April 23, 1999.

The present invention relates to a novel process for making paper basedon cellulose fibre in sheet form, in which a novel retention systemcomprising bentonite and a cationic galactomannan is used to improve inparticular the retention of the mineral fillers incorporated. Thepresent invention also relates to a process for making paper with theuse of a retention system which substantially improves the draining,i.e. the speed with which water flows from the fibre suspension.

In addition, the mechanical properties of the paper obtained accordingto the process of the invention are improved, for example the rigidityand the tear strength, as well as other properties such as thewhiteness. Furthermore, the retention system according to the inventioncan have advantages as regards the quality and recyclability of thewhite waters derived from the papermaking process, as well as of thepapers broken during the manufacturing process.

Papermaking poses several problems. One of the overriding singleconcerns is to reduce the cost of paper by reducing the amount ofcellulose fibres in the paper pulp composition. Another approachconsists in reducing the concentration of waste water on account of theincreasingly strict environmental constraints.

Papermakers have proposed various means to reduce the cost of papers andto try to improve their properties. One of the approaches used consistsin adding inexpensive mineral fillers to the papermaking process toreplace the fibre. Moreover, certain mineral fillers are specificallyused to improve certain properties of paper. Thus, for example, titaniumoxide is used in its anatase and/or rutile forms to improve the opacityof papers, in particular in the case of laminated papers.

Unfortunately, the addition of mineral fillers which aremicrometre-sized particles comes up against the problem of retention:during the formation of the sheet on the paper machine wire, the mineralparticles have a tendency to pass through this wire, which gives rise tocharged white water circuits. This poses problems as regards thetreatment of the broke as well as the sheet quality.

At the present time, the prior art proposes the use of retention agentsto reduce the problem of the lack of retention. For example, EP 490 425A1 proposes a twin system based on anionic inorganic particles and on acationic carbohydrate polymer modified with aluminium, the cationicpolymer being either a cationic starch or a cationic galactomannan.

However, many solutions proposed to date are not economically viable toallow their use for the preparation of any type of paper. The reason forthis is that certain retention agents or retention systems, such asthose containing a carbohydrate polymer modified with aluminium, arecomplex and expensive products, which thus does not allow them to beused for products of ordinary quality.

The Applicant has now developed a novel papermaking process using anovel retention system which considerably increases the retention ofmineral fillers, fibres and other materials in the sheet of paper.

Another object of the invention is to propose a retention system and apapermaking process in which the properties of the paper obtained,including, for example, the opacity yield of the mineral fillers, thetear strength, the whiteness and other necessary properties, areimproved, optimizing the use of mineral fillers. Needless to say, theoptimization takes place as a function of the type of filler used.

Another object of the invention is to propose a paper with a highconcentration of mineral fillers, which has an acceptable tear strengthand other acceptable characteristics.

Another object of the invention is to propose an economically viablealternative retention system which does not require the use of complexand expensive products.

Other objects and advantages of the invention will emerge on reading thedescription below and in particular in the tests, tables and figuresillustrating various characteristics of the invention.

The present invention is based on the development of a retention systemand the papermaking process using it, which markedly improves theretention of mineral fillers and of other characteristics of paper andwhich optimises the action of the mineral fillers present in the paperpulp.

The increase in the retention of the mineral filler and fines in thecontext of our papermaking process attenuates the problems ofcontamination of the white waters.

The present invention thus relates to a process for making paper byforming and drying an aqueous paper pulp containing cellulose pulp andmineral fillers, in which a retention system comprising a cationicgalactomannan containing at least two vicinal groups and a bentonitesuspension is incorporated into the stock pulp before formation of thesheet.

The amount of solids in the retention system is generally from 0.02% to5% by weight, preferably from 0.1 to 1% by weight, relative to theweight of the paper pulp or stock pulp.

The bentonite/galactomannan ratio should be between 1 and 10 by weight,and this ratio is preferably between 2 and 6, depending in particular onthe degree of substitution of the galactomannan.

As regards the bentonite suspension, this is understood as being abentonite suspension consisting of any type of commercial productreferred to as bentonite or as bentonite-type clay, i.e. anionicswelling clays such as sepialite, attapulgite or, preferably,montmorillinite. By way of example, the bentonites described in U.S.Pat. No. 4,305,781 are suitable for use in the context of the invention.

The montmorillonite clays that are suitable include Wyoming bentonitesand soapy earths. The clays may or may not be chemically modified, forexample by alkaline treatment to exchange the calcium of bentonite foran alkali metal.

The swelling clays are usually metal silicates comprising a metal chosenfrom aluminium and magnesium, and optionally other metals, and the ratioof silicon atoms to metal atoms at the surface of the clay particles,and generally within their structure, is from 5/1 to 1/1. For mostmontmorillonites, the ratio is relatively low, the metal beingessentially or totally aluminium, but with a small amount of magnesiumand occasionally with, for example, a small amount of iron. However, inother swelling clays, all or some of the aluminium is replaced withmagnesium and the ratio may be very low, for example about 1.5 forsepialite. The use of silicates in which some of the aluminium has beenreplaced with iron appears to be particularly desirable.

The aqueous suspension is generally prepared by dispersing the bentonitepowder in water. The amount of bentonite contained in the saidsuspension is chosen such that the final weight percentage of bentoniterelative to the weight of the paper pulp will be between about 0.1% and5%. The viscosity of the bentonite suspension is generally less than 500mPa.s (measured using a Brookfield viscometer at 100 rpm).

The size of the bentonite particles is preferably such that at least 90%are less than 100 microns, and preferably at least 60% are less than 50microns (size of the dry particles). The surface area of the bentonitebefore swelling is preferably at least 30 m²/g and generally at least 50m²/g, typically 60 to 90 m²/g, and the surface area after swelling ispreferably from 400 to 800 m²/g. The bentonite advantageously swells byat least 15 to 20-fold. The size of at least 90% of the particles afterswelling is preferably less than 2 microns.

Commercial products which will be mentioned as non-limiting examplesinclude the products Opazil AF and Opazil ADV from the companySudchemie.

The cationic galactomannan according to the invention does not need tobe modified with aluminium; it is preferably selected fromgalactomannans comprising at least two vicinal hydroxyl groups, inparticular cationic guars. As regards the guars, it has been noted thattheir reactive centres are particularly accessible, which makes itpossible to use only small amounts of them to achieve a satisfactoryeffect.

The base guar in the cationic guar is of natural type. The natural guaris extracted from the albumen of certain plant seeds, for example fromCyamopsis Tetragonalobus. The guar macromolecule consists of a linearmain chain constructed from b-D-mannose monomer sugars linked togethervia (1-4) bonds, and a-D-galactose side units linked to the b-D-mannosesvia (1-6) bonds.

The preparation of cationic guars is known per se. By way of example,cationic guars are formed by reaction between hydroxyl groups ofpolygalactomannan and reactive quaternary ammonium compounds.

The degree of substitution of the cationic groups of guar is generallyat least 0.01, and preferably at least 0.05, and can be up to 1.0. Inthe context of the invention, a suitable range extends from 0.08 to 0.5.It is assumed that the molecular weight of guar gum ranges from 50,000to 3,000,000, and is generally about 2,000,000.

When the retention system is used with cationic guar as one of thecomponents, the mineral fillers are retained to a large degree in thefinal product and the paper produced is stronger than the paper obtainedfrom a process without a retention system.

Commercial products which will be mentioned as non-limiting examplesinclude the products from the series Meprobrond 110, Meprobond 9806,Meprobond 109, Jaguar C-13-S, Jaguar C-14-S, Jaguar C-15, Jaguar C-17and Jaguar C-162 from the company Meyhall and from the company RhodiaChimie, and the products Guar CAT 10 from the company Cesalpinia.

Depending on the case and/or on the nature of the galactomannans, thelatter will be formulated in the form of aqueous solutions.

The mineral fillers used in the process are of varied nature and arechosen in particular as a function of the type of paper manufactured andits future use. The mineral filler material which can be used comprisesany common mineral filler whose surface is of at least partially anionicnature.

Among the mineral fillers which will be mentioned as non-limitingillustrations are kaolin, clay, chalk, calcium carbonate, titaniumdioxide and bentonite, and a mixture thereof.

The mineral fillers are normally added in the form of an aqueousdispersion at suitable concentrations that are appropriate for the typeof paper manufactured.

Many commercial products can be used as mineral fillers for papermaking.Non-limiting examples which will be mentioned include the kaolin fromthe company ECC, the calcium carbonate Omyafill from the company Omyaand Calopake from the company Rhodia Chimie, the titanium dioxideFinntitan from the company Kemira and Rhoditan from the company RhodiaChimie.

The possibility of adding mineral fillers to the paper pulps is limitedby factors such as the retention of the fillers on the wire, thedehydration of the paper pulp on the wire and the wet and dry strengthof the paper obtained.

Now, in accordance with our invention, the problems mentioned above dueto the addition of these fillers may be overcome or consequentlyeliminated by using our retention system, which also makes it possibleto add higher than normal proportions of these fillers in order toobtain special properties in the paper produced.

Thus, using the retention system of the invention, it has becomepossible to produce a paper which contains more fillers while at thesame time maintaining its mechanical properties. Hence, by means ofthis, the mechanical properties of the paper, including the modulus ofelasticity, the tensile index, the absorption of tensile energy, etc.have values equal to or even greater than those achieved previously withpapers obtained from conventional paper pulps in which a retention agentof the prior art is optionally used.

The sheet, after drying, has greatly improved strength characteristicswhen the process according to the invention is used. It has also beenfound that when mineral fillers such as those mentioned above and thelike are used in the pulp, these mineral fillers are efficientlyretained in the sheet and furthermore do not have an adverse effect onthe strength of the sheet, unlike the sheets obtained by a manufacturingprocess without a gelling system according to the invention.

Although the mechanism which takes place within the stock pulp duringthe formation and drying of the paper in the presence of the retentionsystem is not totally controlled, it is thought that the retentionsystem forms a combination with the fibres and with the fillers to forma complex flocculent matrix.

Specifically, the manufacture of the sheet of paper necessarily proceedsvia a draining step which can profoundly modify the structure of thecolloids as well as their distribution. The changes in structure of theaggregates of fillers on draining affect the level of retention of thesefillers as well as the opacity of the paper obtained. Thus, in thepresence of the retention system of the invention, during draining, aflocculate is formed within the cellulose resin which imprisons thefillers to preserve the properties of the particles in suspension duringthis critical stage.

The components of the retention system are added to the papermakingdevice as a mixture or separately. However, according to a preferredembodiment variant of the invention, the optimum results are obtainedwhen the retention system, based on bentonite and galactomannan, isformed in situ in the paper pulp.

This can be carried out advantageously by, in a first stage, addinggalactomannan in the form of an aqueous solution and separately addingthe aqueous bentonite solution to the pulp in a mixing tank or in apoint in the device in which there is suitable stirring, such that thetwo components are dispersed with the components forming the paper andthus act simultaneously with each other and with the components forforming the paper.

It has been found that, in a papermaking process using the gellingsystem described in the invention, the pH of the stock pulp is notoverly critical and is generally less than 11 and preferably between 5and 9.

Other chemical additives of the paper can be mixed into the stock pulp,such as antifoams, bonding agents, etc. In this respect, it is importantto ensure that the content of these other agents does not hinder theformation of the flocculent matrix, and that the content of agent(s) inthe recycled white water does not increase too much up to a point whereit would hinder the formation of the flocculent matrix. It will thus bepreferable to add the agent(s) at a point in the system after theformation of the flocculent matrix.

The improvements due to the retention system are observed with an effectof the same magnitude both with chemical pulps and with mechanical andthermomechanical pulps.

From the research and studies carried out, it appears that theprinciples of the present invention can be applied to the manufacture ofany type and quality of paper. Mention will be made, for example, ofwriting printing papers, wrapping papers and laminated papers.

Among the possibilities for preparing a paper type, writing printingpaper is one of the routes which gives very positive results, i.e.increased retention of fillers and improved mechanical qualities of thepaper. In this case, the filler used is mainly calcium carbonate.

The amount of retention system to be used varies according to thedesired effect and the characteristics of the particular componentswhich are chosen in the preparation of the said system. For example, fora bentonite with given characteristics in a retention system, if thissystem contains cationic guar gum with a D.S. of 0.03 instead of a D.S.of 0.07, more retention system will be needed.

The examples and laboratory tests below illustrate, in a non-limitingmanner, advantages and properties associated with the use of theretention system according to the invention for the preparation ofpaper. Good results are obtained with the retention system according tothe invention, although the galactomannan is not modified withaluminium.

The use of the retention system according to the present invention iscompared in particular with the uses of guar alone, of abentonite+starch system and of a bentonite+polyacrylamide system.

Tests

The retention performance qualities are measured essentially by twoparameters:

the retention: amount of fillers retained on the sheet of paper, or forthe total retention, total amount of fine particles retained on thesheet,

the draining: which characterizes the speed with which water flows fromthe fibrous suspension.

These two characteristics can be measured by means of various methods:

“BRITT Jar” method: this measures the chemical retention (total andfillers),

“Shopper-Riegler” method: this measures the chemical retention and thedraining.

Other parameters can be measured by the following methods:

“Rapid Köthen” method: this gives a measurement relative to theformation of the sheet (DIN standard 54358).

Measurement of the tear strength using the “Lloyd 500 Strength Tester”machine.

So-called “Britt Jar” method

This method consists in measuring the chemical retention of the fillerswhile avoiding the formation of the fibrous wad which is responsible formechanical retention by means of a filtration effect. In the context ofour tests, the dissolved cationic guar is added, in a first stage, to1000 ml of the fibre dispersion kept stirring at 500 rpm, followed bythe addition, in a second stage, of the bentonite suspension. The first200 ml are then removed through a screen. By determining the respectiveamounts of fibres and fillers which have passed into the filtrate, theoverall (fibres+fillers) retention values and filler retention valuesare obtained by calculation.

This method of measuring the retention is described by K. Britt and J.E. Unbehend in Research Report 75, 1/10 1981, published by Empire StatePaper Research Institute ESPRA, Syracuse, N.Y. 13210, USA.

For the measurements, a filtration jar equipped with a 125 P aperturegrille with an aperture size of 75 μm was used.

So-called “Shopper-Riegler” method

This Shopper-Riegler method is used according to NFQ standard 50-003.

The cationic guar used has a D.S. equal to 0.1 and the bentonite used isthe product Opazil from the company Sud Chemie.

EXAMPLE 1

Retention

This example shows the chemical retention btained by applying the BrittJar test.

Different formulations with variable percentages of cationic guar andbentonite were prepared (see Table 1 below). The percentages are givenon a weight basis relative to the weight of the paper pulp.

i) Preparation of the stock suspension and dilution

The fibre mixture consists of 60% by weight of Eucalyptus chemical pulpand 40% by weight of sulphated long-fibre chemical pulp. This mixture isobtained by refining in a Rieth Hollander machine up to 24 SR in orderto obtain a pulp with a density of 3%. This pulp is then diluted to 0.5%with a pH of about 7.

Before diluting the pulp, 40% by weight of CaCO₃ as filler and 3% byweight of polyaluminium chloride are added to this mixture withstirring. The stirring is maintained throughout the operations so as toensure ideal homogeneity when samples are taken.

ii) Britt Jar test

1000 ml of the suspension mixture prepared are taken. These 1000 ml areintroduced into the Britt Jar with stirring using a helical-type paddleand fitted with a 125 P 75 μm grille.

The stirring speed is about 500 rpm.

The guar is added, followed by stirring for 60 seconds. Bentonite isthen introduced (in the tests comprising this product). Stirring is thencarried out for 15 seconds.

200 ml of the mixture are removed by gravity.

iii) Filtration

The 200 ml removed are then filtered through a Büchner funnel withWhatmann No. 42 filters (ashless filters predried for 1 h at 105° C. andthen weighed to±0.0001 g).

The filtration residue is then removed carefully, dried for 1 hour at105° C. and then cooled in a desiccator and weighed (±0.0001). Thisallows calculation of the overall degree of retention.

The residue thus dried on a filter is then calcined (according to theNFQ Ash Content standard No. 03-047) to give the degree of retention offillers for the mixture.

iv) Calculations and results

The overall retention is calculated using the following formula:$\text{Overall retention} = {\frac{\left( {{W1} - {W2}} \right)}{W1} \times 100}$

W1=Weight of the mixture (fillers+fibres) in the initial sample removed.

W2=Weight of the residue from the filtered and dried 200 ml sample.

The results are given in Table 1.

TABLE 1 % Increase in % of retention of Formulations % of guar bentonitefillers A 0.05 0 10 B 0.075 0 15 C 0.1 0 21 D 0.2 0 27 A′ 0.05 0.25 29B′ 0.075 0.25 31 C′ 0.1 0.25 33 D′ 0.2 0.25 35

EXAMPLE 2

Draining

This example shows the draining calculated according to the modifiedShopper-Riegler method.

Different formulations with variable percentages of cationic guar andbentonite were prepared (see Table 2 below). The percentages are givenon a weight basis relative to the weight of the paper pulp.

i) Modified Shopper-Riegler test

The suspension of fibre mixture used is identical to that in Example 1.

The guar and bentonite are added and mixed in the Britt Jar containing1000 ml of the suspension mixture prepared in an identical manner tothat of Example 1.

The 1000 ml are then transferred into the tank of the Shopper-Rieglermachine. The time required to drain 600 ml of solution is calculated.

ii) Calculations and results

The time measured in seconds is that of the degree of draining.

For the results, see Table 2.

TABLE 2 % Increase in % of % Increase in Formulations % of guarbentonite draining A 0.05 0 −4 B 0.075 0 −8 C 0.1 0 −8 D 0.2 0 −5 A′0.05 0.25 8 B′ 0.075 0.25 16 C′ 0.1 0.25 23 D′ 0.2 0.25 36

EXAMPLE 3

Comparison with Polyacrylamide and Cationic Starch

i) Composition of the stock suspension and dilution:

The fibre suspension is a mixture containing 30% long fibres, 30% shortfibres, 30% coated broke and 10% CaCO₃, and its pH is 7. This mixture isobtained from a high-density (3.5%) system of a paper machine.

ii) Products used:

0.075% cationic guar of D.S.=0.1 (Meprobond 9806).

0.050% of high molecular weight polyacrylamide (Percoll 292 from thecompany Allied Colloids).

0.500% of cationic starch of D.S.=0.045 (Hicat from the companyRoquette).

0.300% of bentonite (Opazil).

iii) The order of addition of the products is identical to that inExample 1.

iv) The measuring methods are identical to those of Examples 1 and 2(Britt Jar and Shopper-Riegler). In addition, the tear strength ismeasured.

v) Results: see Table 3.

TABLE 3 Cationic Guar + Polyacrylamide + starch + % Change Guarbentonite bentonite bentonite Retention +31 +58 +62 +46 Draining −10 +33+35 25 Tear +10 +13 +2 +15 strength

What is claimed is:
 1. A process for making a sheet of paper from astock pulp by forming and drying an aqueous paper pulp containingcellulose pulp and mineral fillers, comprising the step of incorporatinga retention system comprising (i) a cationic galactomannan containing atleast two vicinal hydroxyl groups and not modified with aluminum, and(ii) a bentonite suspension into the stock pulp before formation of thesheet.
 2. A process according to claim 1, wherein the retention systemhas an amount of solids of from 0.1% to 5% by weight, relative to theweight of the paper pulp.
 3. A process according to claim 1, wherein thegalactomannan has a degree of substitution of at least 0.01.
 4. Aprocess according to claim 3, wherein the galactomannan has a degree ofsubstitution of at least 0.05.
 5. A process according to claim 4,wherein the galactomannan has a degree of substitution of up to 1.0. 6.A process according to claim 1, wherein the mineral fillers are clay,chalk, calcium carbonate, titanium oxide, or bentonite.
 7. A processaccording to claim 1, wherein the stock pulp has a pH maintained between5 and
 9. 8. A process according to claim 1, wherein the retention systemhas a solid content of from 0.02% to 5% by weight, relative to theweight of the stock pulp.
 9. A process according to claim 1, wherein theretention system presents a bentonite/galactomannan ratio between 1 and10 by weight.
 10. A process according to claim 9, wherein thebentonite/galactomannan ratio is between 2 and
 6. 11. A processaccording to claim 1, wherein the retention system is formed in situ inthe paper pulp.
 12. A process according to claim 1, wherein theretention system is formed in situ in the paper pulp.
 13. A processaccording to claim 11, wherein the retention system is formed in situ byadding, in a first stage, galactomannan in the form of an aqueoussolution and adding, in a second stage, aqueous bentonite solution tothe paper pulp in a mixing tank or at a point in the device in whichthere is stirring.
 14. Laminated paper, writing printing paper orwrapping paper made by the process of claim 1.